1. Field of the Invention
The present invention relates to an apparatus and a method for generating a return-to-zero (RZ) signal, and more particularly, to an apparatus and a method for generating an RZ signal at low cost using two optical lasers without using two optical modulators in an optical transmitter.
2. Description of the Related Art
In general, a laser is directly modulated to generate a return-to-zero (RZ) signal of 2.5 Gbps or less. A stable RZ signal of 10 Gbps cannot be obtained through a direct modulation of a laser. Therefore, two external optical modulators are generally used. In the case of such an RZ modulation, a first optical modulator generates a pulse string, and a second optical modulator modulates data to obtain an RZ signal. Thus, the use of the two optical modulators incurs large costs. Moreover, a low cost method is required to be used in an optical network or the like.
FIG. 1 is a diagram illustrating a method of generating an RZ signal using two modulators according to the prior art. Referring to FIG. 1, an optical laser 110 applies a continuous wave (CW) laser light to a first Mach Zehnder (MZ) modulator (referred to as a optical modulator) 120. The first optical modulator 120 receives a clock signal of 5 Hz or 10 GHz from a radio frequency (RF) input port to output a pulse string of 10 GHz. If the optical modulator 120 receives the clock signal of 5 GHz from the RF input port, the first optical modulator 120 receives an RF signal having an amplitude doubling Vpi (a peak of input voltage) of the first optical modulator 120 to generate a pulse string of 10 GHz. If the first optical modulator 120 receives the clock of 10 GHz, the first optical modulator 120 receives an RF signal having an amplitude equal to Vpi to generate a pulse string of 10 GHz. However, a pulse width of the pulse string depends on a transfer function of the first optical modulator 120. Thus, it is difficult to obtain a pulse string having pulses having a small pulse width. Accordingly, if the pulse string is modulated into data, cross talk is highly likely to occur between neighboring bits. Thus, a probability of an error occurring is increased.